Eosinophils are closely associated with mast cells in immediate hypersensitivity reactions. Activated eosinophils generate H2O2 and release granule components such as peroxidase (EPO) into the extracellular fluid. EPO, which is a strongly basic protein, also binds firmly to the matrix of the isolated mast cell granules (MCG) with retention and augmentation of peroxidatic activity. The cytotoxic effect of the MCG/EPO complex-H2O2-halide system will be assessed on various target cells such as schistosomula. The target cells will be labeled with 51Cr and the release of 51Cr from the cells will used to measure cytotoxicity of the peroxidase system. Mast cells also contain small amounts of endogenous peroxidase in their granules and the toxicity of the MCG peroxidase system on target cells will also be examined. The effect of hemeprotein inhibitors on the peroxidase system will be used to test for specificity of the reaction.